Three prime features of the martian landscape are volcanism and volcanoes, linear structures indicating some tectonism (but not plate tectonic movements of crust), and impact craters. It appears that the bulk of the martian rock materials are volcanic, probably of low silica (basaltic) nature. The martian crust in its broad or first order state can be subdivided into old, heavily cratered terrain in the southern hemisphere and younger, less cratered terrain, with abundant volcanic features in the northern one. The martian surface displays both conventional (i.e., similar to Earth’s) and exotic terrains and erosional/depositional processes. One of great importance is sinuous channels that are much like river beds on Earth. This holds big implications: that Mars once had, and may still have, some water, probably for the latter part of its history in subsurface storage. Mars may also have had in the past enough water to develop large lakes or ocean-sized bodies. Like the Moon, Mars has large-scale erosional and depositional units which give it a general stratigraphy that allows geologic maps of its surface to be drawn.

A plethora of spectacular surface images have accrued from the Mariner 9 and
Viking missions. Below, we show only a few, to whet your intellectual appetite,
but for the curious, consult these two references for many more pictures: The
Geology of Mars, T.A. Mutch et al., 1976, Princeton University Press and
Viking Orbiter Views of Mars, C. R. Spitzer, Ed., 1980, NASA SP-441.

A reminder: As has been alluded to before in this subsection, Mars seems to consist of two dominant terrains - the northern half is mostly a volcanic plains with numerous volcanic phenomena, but is only moderately cratered; the southern part of Mars also consists of igneous (probably volcanic) rock crust but that is much more heavily cratered and has not seen major surface reworking or lava paving. Tectonic features - almost entirely faulting (probably tensional) are found over the entire planet.

No firm evidence of folding in the Mars rocks, which in many places are layered, has been found to date, although inclined strata have been noted in crater walls. This suggests that compressional force activity is very uncommon on Mars, i.e., plate tectonics as acts on Earth has not taken place. Faults are frequent, however, indicating some extensional forces have pulled the martian crust apart in places. Directions of tension have changed over time at given locations. The first image on this page shows three such tensional faults or grabens:

This scene clearly indicates the "Law of Cross-cutting" included in the basic review of Geology in Section 2. The law indicates relative ages. The oldest faulting is indicated by the structure starting in the upper left that slants towards the lower right. It is cut by the fault running upward to the right. The youngest fault, cutting this second fault, appears to the right of the first and appears the freshest (note the sand dunes within it).

Linear faults on Mars are fairly commmon. This image shows a set of parallel faults; along one is a chain of pit craters (surface material sinks into the fractures). Some have interpreted this alignment and association with fresh-looking normal faults as a sign of recent movement in the martian crust, causing "marsquakes" as a consequence:

Many of the scenes depicted below that illustrate the topics listed in the page heading occur in and east of the Tharsis region of Mars. To establish these in their physiographic context, here is a MOLA derived topographic map that includes some of the landforms we will visit:

We start with the greatest trench in
the ground ever discovered in the solar system. Look at this Mariner 9 mosaic,
extracted from the near hemisphere view, here centered on
the most conspicuous feature on Mars: the Valles Marineris which extends nearly
4,000 km (2,486 mi) and attains depths between 2 and 7 km (1.25-4.35 mi). When the outline of the 48 contiguous United States is overlain on mosaic, the eastern edge of Valles Marineris touches the Outer Banks of North Carolina and its western edge reaches to Central California.

Part of the Valles Marineris around Candor Chasma is shown in color in this Viking image:

This is what you would see if flying over the Valles (canyon). The image is made by combining a Viking view with MOLA data; there is no vertical exaggeration.

The canyon
is actually a series of structural troughs, produced by faulting, radial to
the Tharsis bulge to the northwest, which rises some 11 km (6.8 mi) above the
surrounding plains, on which are the three dark-shield volcanoes , named on
the preceding page. These volcanoes reach about 10 km (6.2 mi) above the bulge.
A look inside the canyon wall, along a segment called South Candor Chasma, conveys
the sense of steep slopes, perhaps furrowed by water erosion, and basal landslide
deposits.

Another landslide into Valles Marineris appears here; below it is hummocky terrain often found in the deposits at the slide's foot.

A close-up of a landslide in Valles Marineris
gives details of the massive debris pile-up, as material pulled away from the
steep canyon wall.

Along some edges of Valles Marineris are what appear at first to be tributary valleys. But they don't enter at levels equivalent to the floor base. They seem to criss-cross in a pattern that suggests tectonic control. One proposed explanation has them as due to subsurface water sapping.

19-39:
What general explanation can be given for the formation of Valles Marineris?
ANSWER

Other tensional grabens are found
in various parts of Mars, especially in the newer terrains. These can occur
in intersecting networks, such as below which portrays Noctis Labyrinthus in
the northern Tharsis region. This tecto-morphological feature is also called
fractured terrain.

Sets of subparallel fractures cuts across the terrain on the flank of the Tharsis region (Tharsis is a huge upbulge of martian crust more than 4000 km across and 10 km higher than surrounding lowlands at its top; Olympus Mons and the Tharsis volcanoes attest to it volcanic nature). In overall pattern, the sets are radial to the Tharsis apex. Here is one such set which cuts across older craters (but several younger craters superpose on the fractures).

As is often true for volcanic terrains
(such as the East African Rift), sets of close-spaced parallel grabens (fault-bounded
downdrop blocks of crust) related to tension induced by loss of support after
lava withdrawal have been found also on Mars:

At the other extreme, short fractures may appear as isolated gashes, as seen here. These may be incipient or early stage breaks in a surface undergoing only moderate tensional stress.

Next, we switch our review to volcanism on Mars. In the Tharsis bulge region, some 4000 km across and 10 km above the mean martian elevation, are four of the biggest volcanoes in the Solar System. The huge structure alone in the western end of the Tharsis region is known as Olympus Mons, which is a broad shield volcano (now dormant), many times the area
and volume of the big island of Hawaii, which consists of basaltic outflows
from several major vents. Olympus Mons has a median diameter of 625 km (388 mi) and
a height of 25 km (16 mi). We show first the famous discovery image from Mariner
9 (top), then a color version from Viking (center).

Eye-pleasing as the color image is, this black and white Viking version shows more information about lava and other features because of the contrast:

Olympus Mons is the largest volcanic structure known on any of the planets. In fact, the major volcanoes of the Tharsis region are all huge by Earth standards. This is self-evident when the next two illustrations are examined. The first plots Olympus Mons and its three Tharsis companions (see below) on a map of the United States. Only Mauna Loa on the Island of Hawaii can compete with the three. When plotted as cross-sections the size of Olympus Mons is even more awesome.

Olympus Mons' elliptical central caldera is 80 km (50 mi) in major axis. Here is first a color view looking down from the Mars Express spacecraft and then a stark black and white image made by the Mars Global Surveyor which emphasizes the relief aspect of the caldera:

A steep cliff up to 6 km (3.7 miles) high surrounds Olympus Mons, and stands out in the
perspective view (below), derived by combining a Mars Express image with topographic data obtained from laser altimeter data.
Scientists still debate the origin of this cliff, but some of them cite it as
evidence of an escarpment resulting from wave erosion by an ancient (now vanished)
ocean that may have covered at least part of Mars. Note the distinctive grooved
terrain (volcanic origin?) beyond the volcano's edge.

Mars Express imagery has also been manipulated to produce a view of the scarp (cliff) as though seen from the sloping plains beyond it:

Along the scarp these flows have spilled over into a moat-like shallow depression. This indicates that volcanism continued on Olympus Mons after scarp formation.

19-40: This steep cliff around Olympus Mons is peculiar and not characteristic of
terrestrial shield volcanoes. Speculate on a possible origin.
ANSWER

Once above the fringing escarpment, the slope of Olympus Mons is gentle - 1 to 3 °. However, this incline allows lava flows to move downslope, as shown in this example from Mars Orbiter:

There are three volcanoes lined up in the Tharsis Montes area east of Olympus Mons; from south to north they are Arsia Mons, Pavonis Mons, and Ascraeus Mons. Each is a large shield volcano with a well developed central caldera. Typical is Arsia Mons, about 300 km (200 miles) wide at its base:

A side view made by combining a Viking image with MOLA elevation data gives this impression of Arsia Mons:

One of the best formed volcanoes in Bilbis Patera, which lies between Olympus Mons and the Tharsis group. As seen here by Viking Orbiter 1, the base of the volcano is about 100 km (62 miles) in diameter. The caldera is large compared to the total size, like some volcanoes in the Galapagos Islands off the Ecuador coast.

The largest volcanic complex on Mars is the great bulge in the northern Tharsis region known as Alba Patera, only a few kilometers high but 1600 km
(1000 miles) in longest dimension.
As such, it rates as the largest shield volcano in the Solar System.It is broken by a series of concentric fractures and a set of elongate, subparallel fracture grabens, as seen in this Mariner mosaic.

At Alba Patera's top are an older summit
caldera (left) and a smaller, more recent one. Note the lava flows descending
its gentle slopes.

Alba Patera's flanks show numerous overlapping flows, indicating multiple periods of lavas extruded from tube outlets both at the caldera and along the slopes. In this Viking Orbiter image, the flows are flat-topped but steep-sided; volcanic ridges appear in the lower left.

Some lava flows are regular, smooth-to-rough surfaced, and with definite steep fronts, similar to those observed on the island of Hawaii. Here are three examples:

Lava flows issue from volcanoes, vents, and fissures. A narrow fissure can be filled with lava that hardens. As erosion removes its surroundings, the lava mass stands above the surface as a dike, as shown here:

Smaller, more conventional volcanoes on Mars are known as tholii. An example is Ceraunius; even smaller are equivalent to large volcanic cones found on Earth:

In contrast to these volcanoes, which are upward conical prominences, are the downward indentations or craters that can be either volcanic or impact. Both are typical of martian terrains. Extensive impact cratering was
observed by Mariner 4, which sent back the first ever images (21 all told)
taken of another planet's surface (one seen in the top photo below) by a space
probe that approached to within 9800 km (6086 miles). As imaged the next year
by Mariner 6, the Sinus Sabeus region of the southern highlands (bottom) preserves
typical impact craters in the ancient terrain that apparently has not been
extensively resurfaced by lavas. Note that none of the larger craters in this
view have central peaks.

Mariner 9 and the Vikings confirmed
that a large fraction of the (older) martian surface mainly in the southern
hemisphere remains heavily cratered. This is evident in this sketch drawing
from Mutch et al., The Geology of Mars, 1976 in which all craters larger
than 15 km are positioned.

A recent study made by a colleague of the writer (NMS), Dr. Herbert Frey of NASA Goddard - assisted by his teen age daughter Erin - has led to a map of the distribution of large surface-visible plus now buried impact structures that nevertheless show circular surface manifestations. The latter have been located using the MOLA laser altimetry data.

One can argue that this landscape
has many similarities to the still cratered Earth in its early stages before
extensive water had collected into major oceans. Likewise, buried impact structures can be discerned on the lunar surface. These have since been covered by lunar ejecta. This may mean that the martian surface is also covered by ejecta deposits that spread over older craters.

Some of the martian impact
structures retain well-preserved ejecta blankets that display prominent lobes,
such as seen here around the crater Yuty. The ejecta was probably fluidized
by vaporization of carbon dioxide-rich ice lying just beneath the surface.

One type of impact crater is different from those on the Moon, Mercury and Venus in that the edge of the ejecta blanket has a steep scarp, evident in the Viking image below, or even a peripheral rise called a rampart. This type is called a pedestal crater.

On Mars many of the younger craters still preserve their ejecta blankets, as exemplified here:

This next crater is small, young, and shows most of the same features as do terrestrial craters. Located in Terra Meridiani, this crater is 2.6 kms wide (1.6 miles; rim to rim), has at least 1 nested slump zone in its interior and a distinct exterior ejecta blanket, and has exposed what appears to be internal layering of the martian surface units. The image was made by the Mars Global Surveyor.

This type of central (interior) layering, almost certainly sedimentary (see pages 19-13a and 19-13b) also appears in the 2.3 km (1.5 mile) wide Schiaparelli crater in the Chrysae Basin, seen below. The layering appears horizontal:

These observations of sedimentary-like crater interior floors and walls (layering is also discussed on the next page) seem rather mysterious to the writer (NMS). On Earth, craters that still retain their original rims (almost?) never show the bedrock below the final crater excavation wall. Yet this is common in martian craters with initial walls intact. Martian planetologists have suggested removal by erosion (they mean almost certainly wind erosion). The writer speculates on an alternate cause: the lower martian gravity allow nearly complete escape during crater formation of the bulk of the ejecta; the floor remains exposed because in the smaller craters slumping has not destroyed the walls.

An interesting "inversion" of landforms displaying layering occurs in this small mound analogous to a circular "butte" on Earth. It shows layers that form ledges that decrease in diameter going up to the feature's peak. The mound is in a crater floor and may be a residual remnant of layered crater fill (wind deposits; lake beds?).

Still another large impact crater,
Poona, has a remarkable uniform set of rays, equispaced over the full 360°
around the rim:

This small crater (below) shows a distinct pattern of dark rays. Because martian winds are continually altering the surface, both removing and covering up debris, the crater (and those above with lighter-toned rays) can be young - age estimates have ranged between a few thousand and a few million years.

This next Viking scene, in the
southern Highlands, seems to have both impact and volcanic craters. Some without
ejecta beyond their rims, especially the elliptical one, are calderas. Several
others have aspects more characteristic of degraded impact structures. This
was an active region, with channels (either volcanic or stream) and other
types of terrain.

Now look at these three craters
(Ulysses Patera):

19-41:
What type(s) of craters are present in this Viking scene (the largest structure
is about 80 km [50 miles] across at its base)? ANSWER

Because of several factors, some martian craters appear as faint rings rather than topographic features raised above the surface. These have been called "ghost" or "stealth" craters. They represent some combination of burial by crater ejecta, wind erosion, dust cover, and ice cover. Here is an example of this last type:

More commonly, the burial by debris and dust is only partial as indicated by this image:

There is evidence that the number of observed impact craters on Mars is less than would be expected if the recent activities (dust transport and deposition, ice relocation, etc.) had not buried the smaller ones. The wind, however, is capable of exhuming such craters, as displayed in this image which also shows the exposed craters to contain some signs of filling by sediment, now revealed as faint layers.

Not all impact craters are circular or slightly elliptical. Strongly elongate craters are found on the Moon. A few such distorted craters are present on Mars, such as the one shown below. The usual explanation is that the impacting body comes onto the surface at a very low or grazing angle, scouring out the surface material as it proceeds forward:

Large, young impact craters are few but conspicuous. Galle Crater is 220 km (138 miles) wide and retains its original rim:

As with the Moon, Mars has a few craters so large that they can be called impact basins. The second largest (600 km; 390 miles diameter) is the Argyre Basin, seen in this Viking view:

The landscape just beyond its rim looks remarkably like parts of southern Utah, except for the pinkish-orange rather than blue sky:

The largest impact basin on Mars, and seemingly the largest in the Solar System, is the Hellas Basin in the southern Highlands. Its diameter is about 2100 km (1300 miles), its depth is almost 9 km (6 miles) and its rim exceeds 1.5 km (1 mile). As seen by Viking in a wide-angle oblique view, the huge size of Hellas dwarfs the rim and interior elevation differences. In this view the Basin appears to have no significant landforms within it.

To emphasize the size of this structure: If all material
excavated from it were to be spread evenly over the 48 continental United States,
a layer of debris some 3.5 km (2 miles) thick would accrue. Below is an enlargement of the map covering this structure.

The floor of Hellas actually shows diverse landforms, some of which appear volcanic in origin (this assumes the basin filled with melt soon after the impact event).

Among features believed volcanic in nature are linear ridges similar to the wrinkle ridges found in lunar maria. Here is a topographic map made from MGS MOLA measurements that includes (in the purple) these ridges and shows the diversity of other landforms.

Mars investigators have speculated that during the early eons of martian time, when the atmosphere was possibly more abundant (thicker, with greater surface pressure), water released by impacts and other processes could be distributed as rainfall. Some think that shallow lakes filled Hellas, Argyre and other large craters for a time.

In some of the above images, and several on pages 19-13a and 19-13b, features that could be described as mountains are displayed. Of course, volcanoes fall broadly into that category. Rims around large craters also are mountainlike. Here is a series of mostly parallel ridgelike prominences that are considered low mountains, found here in a region called Tithonium Chasma:

So, once again we see a planetary body with a great variety of landforms, many caused or affected by impact processes. Some of these are unusual (exotic) including those which may reveal water erosion.

Channels and Exotic Terrains

By now, one should be convinced that Mars is a geomorphologist's Paradise. As with the Moon in the earlier days of exploration, landform identification, with educated "guesses" as to modes of formation, has been the prime approach to mapping and interpreting the martian surface. Mars exhibits a great variety of terrains and landforms types. Most are given terms that have a Latin derivation. An excellent summary with numerous examples of these types is found at The Atlas of Mars web site. Click on the terms in the left column which brings up usually many images each displayed by clicking on its entry phrase. It is well worth your time to spend an hour or so looking at the wide range of landforms recorded at this site.

Some of the big surprises were infrequent
but distinctive sinuous channels, whose morphology is much more similar
to river channels that lava channels. One interpretation holds this morphology
as evidence of sufficient water in the past, in lakes, groundwater or possibly
oceans, to have initiated some sort of hydrologic cycle involving rain storms
and runoff. Most water has since evaporated into space, although small quantities
may remain frozen as underground ice. Nevertheless, significant water activity
has recurred as evidenced by the types of dendritic channeling shown in these
Viking Orbiter images:

The region depicted in the top image covers the Juvenae Chasma and Vedra Vallis.
These are runoff channels, a type confined to the ancient landscapes.
Stream flow is the favored origin, largely by comparing them with terrestrial
counterparts, but multiple lava flows are a possibility. Some channels seem
to originate at craters, which could imply that subterranean sources released
either water or lava, following impact offloading. The drainage pattern in the
bottom image resembles terrestrial patterns found in soft sediments or wind
deposits.

19-42: What
is the argument that the type of channels shown above is not volcanic sinuous
rilles or collapsed lava tubes?ANSWER

Collapsed lava tubes have been found in association with martian volcanoes. Here is one example:

This Viking image shows a channel called Nirgal Valles that looks much like the sinuous rilles described on the lunar surface. Whether this was caused by lava tube collapse or poor fluvial action is not obvious at this scale:

A much higher resolution MGS MOC image of a straight segment of Nirgal Valles display features favoring a fluvial origin over volcanic collapse:

Nevertheless, the resemblance of many of the martian channels to fluvial channels on Earth is particularly evident in the next (MOC) image. Located within the large Newton craters, the dark (windblown sand-filled?), flat-bottomed channels look like some headwater types for streams found on Earth:

The "Jury is Out" on the exact origin of the narrow channels in this MGS MOC image. What can be said is that over much of the depression light-colored wind deposits have been trapped and shaped into dunes resembling large ripples:

A distinctive type of drainage called
outflow channelling are typically broad and deep, creating
canyon-like depressions. A typical example, seen below, is Ma'adam Valles, some 300 km long, which ends in Gusev Crater (far upper right; see page 19-13a):

This type of landform (left image) may have been associated with
catastrophic scouring during abrupt flooding. A similar example on Earth is
the Channeled Scablands of central Washington State in the U.S. that developed
in just a few weeks from rapid emptying of a huge dammed lake after a natural
breakup. Another indication of strong fluid action is a teardrop-shaped landform in Elysium Planitia (right image) a prime example of shaping by streamlining (analogous to aerodynamic sculpturing), in which water flowing from bottom to top has eroded plains material around the rim of
a large crater and has terraced and perhaps redeposited debris towards the pointed
end.

19-43: Present
an argument as to why the teardrop landform was caused by water rather than
wind.ANSWER

If riverlike channels did once carry water over the martian surface, one landform they should produce is a fan deposit made up of the debris carried by the streams until such streams are slowed such as to cause their sediment loads to be dropped. A prominent distributary fan has now been found in an unnamed crater in the southern hemisphere. The delta-like fan is 13 km (8 miles) by 11 km (7 miles) in dimension. Here it is in a Mars Global Surveyor image:

A close-up of a part of this fan shows an anomaly. In the image below, one and perhaps two flat-topped ridges emerge above sculpted out surfaces (exposing layers). These ridges may be made of channel deposits that were more resistant than surrounding deposits so that after general erosion of the fan, these remain as topographic highs:

The consensus as of late 2004 is that many - perhaps most - martian channels were carved by running water at times in martian history when liquid water was much freer to flow in copious amounts over the surface; probably the martian atmosphere was denser in those times. A plot of major channels in the non-polar regions of Mars reveals an interesting pattern:

What seems mysterious about this pattern is that most of the channel systems end in a "blank" (black) part of the map. Thus the majority of systems were independent and did not connect with each other. The presumption is that each channel emptied into a relatively small region of Mars. In such a region the water would connect with a standing body of lake-sized proportions. While this in itself does not rule out oceans, those would have existed before channel-cutting. The question that emerges when trying to explain the widespread distribution of sedimentlike layers over much of Mars is whether these were dominantly lake deposits or could at least some represent a more continuous marine stage.

One thing is now sure. Some process(es) is/are still producing new channels. This image below shows the same smooth (sandy?) plains that ends abruptly in a cliff. On the left, the image indicates no channeling; on the right at a later time, a well-defined small channel network has since formed. What caused this is still conjectural; wind erosion is one suggestion.

Another landform that is almost exclusively formed by water-involved erosion is the mesa (or its smaller form, the butte); on Mars it is termed "mensa". This occurs usually when a more resistant layer series is on top of a weaker, more erodable set. Water penetrates to the lower layers, gradually exposing the surface, and causing the upper layers to diminish in size and extent as undercutting erodes into those layers. A residual series of higher landforms results as remnants of the original upper layers on the stripped surface. Here are two excellent examples of a Mars mesa, the first found in Granicus Valles, the second is called Lunae Mensa:

Light-topped mesas are found near Valles Marineris:

Seen closer-up, mesalike blocks of this whitish material shows faint but distinguishable layering. Such units appear to be erosional outliers or remnants of a once continue sequence of deposits - probably formed by some sedimentary process - found over various regions distributed over large areas of Mars.

Other variants of the mesa landform include remnants of a thick dark unit found above the lake beds in Aram Chaos (see page 19-13a) and flat-topped CO2 ice "mesas" separated by flat pits in the south Polar ice sheet.

A recent paper has presented an alternative to water as the prime liquid medium responsible for the above channels and streamlike patterns. In this view, CO2 (carbon dioxide) is proposed to exist in liquid form and in flowing upon expulsion at the surface brings about the erosional features described as fluvial. A variant of this suggests that liquid water at the times in the past when Mars was warmer may have contained a significant amount of dissolved carbon dioxide ("soda water") that increased its ability to erode.

So, as of mid-2004 what can be said that reasonably affirms the presence of water now and in the past in martian history. On the next page (19-13a and 19-13b) we will learn of the direct observation of sediments that normally require the involvement of water. The discovery of water in polar ice, probably in subsurface lower latitude materials, and in the thin atmosphere all point to survival of small amounts of water today. This water may just be that released by occasional volcanism or by shock-evaporation of incoming comets. The various riverine landforms shown above on this page seem to point to greater amounts of water in the past. One school of thought concludes that past atmospheres were more dense and had a greater water content. Ancient martian atmospheres have probably been progressively depleted of gaseous and liquid molecules, including water, by thermal activity, by gravitational loss, and espesially by the relatively weak but potent solar winds.

The martian surface is amazingly
varied, with landforms of diverse genesis, some probably related to water
action, as you have just seen, and others to tectonic forces, volcanism, and wind, being given descriptive names. Here are some typical examples:

The mishmash of intersecting linear
features, called grooved terrain, in this case may be a complex surface
of eroded ash deposits or possibly joint enlargement of a now buried remnant
of a volcanic lava unit.

Variants of grooved terrain are known as sulci (singular, sulcus). Here is an example seen in a thermal image made by THEMIS:

Similar terrain occurs in the slopes beyond Olympus Mons where the features present are called part of this volcano's aureole. The criss-crossing grooves and ridges seen here are almost certainly tectonic in nature:

In the next image is an example of
polygonal terrain, which resembles, on a huge scale, fractures found
on lava lake surfaces or patterned ground, associated with ice wedging in periglacial
regions (areas adjacent to ice sheets).

The last image dealing with possible water-influenced ground shows fretted terrain,
found usually near cratered terrain, consisting of separated higher mesa-like
units, bounded by scarps and set within lower smooth plains. This terrain may
represent incomplete dissection of older landforms by water and/or wind. Here are three examples:

The next image portrays etched
terrain which consists of shallow depressions likely developed by wind scouring
and deflation of easily erodable unconsolidated surface materials.

Other landform types given distinctive
names (see the map near the top of page 19-11) include:
furrowed terrain, knobby terrain,channeled terrain, and layered terrain. Examples of several of these are shown elsewhere in the Mars subsection. Below is an example of a peculiar terrain found mainly in Hellas Planitia. It is termed colloquially "taffey-pull" terrain. It's formative nature remains uncertain but one interpretation includes the possibility of erosion of hard and soft layers of sediment-like material; this does not quite explain the flow patterns in apparent channels.

19-44: From
the above list, decide which name best fits the terrain shown in the two images below;
ignore the large craters in the first.ANSWER

Some of these exotic terrains can also be calle enigmatic. Lets illustrate this claim by looking at a series of images that center on what was called "White Rock" after its discovery in Mariner 9 images. The feature is a light-toned landmass, strongly embayed, that rises above the floor of the crater named Pollack (seen here in a MGS MOC image) in the southern highlands at a low latitude:

In a Viking black and white image, this feature, which is approximate 12 x 12 km in dimension, indeed has a higher albedo than the crater floor (very dark) and thus stands out as an off-white feature. But comparing it to a Viking color image indicates 1) it has the same reddish surface coating that most of Mars has, and 2) it "whiteness" is largely due to contrast with the floor; its gray tone level is similar to surface materials beyond Pollack's rim.

One of the early interpretations considered it to be ice preserved as a patch in the crater. One investigator proposed this feature to have been ice extruded from depth, much like salt forms in domes and may reach the surface. This was discounted by radiometric measurements that indicated too high a temperature and later measurements that ruled out H2O and CO2. Interest was renewed in White Rock from MOC images taken onboard the Mars Global Surveyor. Consider this next pair that zero in on the several prongs of the feature:

In the upper right of this last image is an enlargement of the teardrop-like features. These seem to show thin layers but their shape is attributed to yardang sculpturing by wind:

This image concentrates on several white prongs and the terrain in between. Of interest are the series of thin, arcuate equi-spaced lines in the dark areas between the ridges in the lower of the paired images. These seem to be controlled by the ridges. My interpretation is that they are regular dune-like markings that may result from martian winds that are directionally channeled by the ridges - but this is speculation.

Mars scientists now interpret this feature to be dissected lake beds, if Pollack was once filled with water. Others propose volcanic ash deposits that collected inside the crater and are being systematically removed (by wind?); no nearby volcanic vent is evident that would account for this. Still other explanations have been proposed. No consensus explanation has been reached at this time.

There is still much to do to properly categorize and explain the surface features on Mars.

An excellent review article, "The Unearthly Landscapes of Mars" by Arden Albee, appearing in the June 2003 issue of Scientific American offers further insights into the surface features of the Red Planet.